Is your background grey but the colors are inconsistent from the lighting? This works for any color background as well. Try using any version of Photoshop for a fast, easy adjustment in a few steps. I recommend having a color check picture on the screen at the same time to make sure you in the same hue level. Image-Adjustments-Replace color

Isolate the background by using the eyedropper and then increase that area using the +plus dropper or sliding the fuzziness bar. Second step is dropping the saturation all the way to the left for grey.